Heat-dissipating substrate for led

ABSTRACT

The present invention relates to a heat-dissipating substrate for LED, comprising a polyimide film, a copper foil or a copper alloy foil which is laminated on one side of the polyimide film, and an aluminum foil or an aluminum alloy foil which is laminated on the other side of the polyimide film, in which the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil is 1.8° C./W or less.

TECHNICAL FIELD

The present invention relates to a heat-dissipating substrate for LED. More specifically, the present invention relates to a heat-dissipating substrate for LED, which is thin, bendable and 3-dimensionally processible.

BACKGROUND ART

In recent years, due to lower power consumption and longer life, the demand for an LED lighting device, which comprises an LED (light-emitting diode) as a light source, has been rapidly growing. However, an LED generates heat while lighting up. When the LED is heated up to a high temperature by the generated heat, the luminous efficiency of the LED may be significantly reduced and the life of the LED may be affected. In the case of high-power and high-luminance LEDs, in particular, the LEDs generate a larger amount of heat while lighting up, and therefore it is more important to dissipate the heat of the LED to avoid a rise in temperature of the LED.

Accordingly, a heat-dissipating substrate, which has excellent heat-dissipating properties, is used as a substrate for LED mounting. The commonly-used heat-dissipating substrate comprises an epoxy resin film, which is an insulating layer, and a copper foil laminated on one side of the epoxy resin film, and an aluminum foil laminated on the other side thereof. In terms of withstand voltage, however, an epoxy resin film having a thickness of about 1 mm, at least about 100 μm or more, must be used, if an epoxy resin film is used as the insulating layer. Meanwhile, because such a thick epoxy resin film has a high thermal resistance, an inorganic filler such as alumina which has high thermal conductivity must be added to the epoxy resin film in a large amount for reduced thermal resistance in order to achieve adequate heat dissipation. However, such a heat-dissipating substrate for LED, which contains a large amount of inorganic filler, may have low processability and the filler may be scattered around during machining (machine-processing), which may cause a problem. In addition, this heat-dissipating substrate for LED cannot be bent and cannot be 3-dimensionally processed (or fabricated). The design freedom in the circuit is increased when a heat-dissipating substrate for LED is 3-dimensionally processible. Accordingly, there is a need for a heat-dissipating substrate for LED having good bending properties.

Patent document 1 discloses an LED lighting device comprising

a substrate body which is formed of a thermoplastic heat-resistant film comprising polyimide in a certain 3-dimensional form,

one or a plurality of surface-mounting type LEDs which is mounted on the substrate body in position, and

a conductive circuit which is installed in either the surface or the back surface of the substrate body, and connects the LEDs to an external circuit and turns on the LEDs,

wherein a heat-dissipating layer made of metal is formed on the surface of the substrate body opposite to the conductive circuit.

Patent document 2 discloses a copper-clad sheet to address the heat dissipation, which is a copper-clad sheet having high thermal conductivity, comprising

a multi-layer polyimide film, in which a base layer of polyimide (X) having low thermal expansion coefficient, and two thin layers of polyimide (Y) comprising the specific imide unit are laminated together and integrated, the thin polyimide (Y) layers being laminated on both sides of the base polyimide (X) layer,

a copper foil, which is laminated on one side of the polyimide film, and

a metal sheet or a ceramic sheet having high thermal conductivity, including an aluminum sheet having a thickness of from 5 μm to 2 mm, which is laminated on the other side of the polyimide film.

In addition, Patent document 3 discloses a laminate to be used for LED mounting and interconnection, which comprises, on a substrate, a copper or aluminum metal layer, a polyimide or adhesive layer adjacent to the metal layer, a copper foil layer, and a liquid or film solder mask layer.

CITATION LIST Patent Document

-   Patent document 1: JP-A-2008-293692 -   Patent document 2: JP-A-2003-71982 -   Patent document 3: WO 2009/073670

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a heat-dissipating substrate for LED, which is thin, and has excellent heat-dissipating properties and withstand voltage, and also has good bending properties, and therefore is 3-dimensionally processible.

Means for Solving the Problems

The present invention relates to the following matters.

(1) A heat-dissipating substrate for LED, comprising

a polyimide film,

a copper foil or a copper alloy foil, which is laminated on one side of the polyimide film, and

an aluminum foil or an aluminum alloy foil, which is laminated on the other side of the polyimide film;

wherein the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil is 1.8° C./W or less.

(2) A heat-dissipating substrate for LED as described in (1), wherein the aluminum foil or the aluminum alloy foil is not subjected to anodization treatment (alumite treatment).

(3) A heat-dissipating substrate for LED as described in (1) or (2), wherein the polyimide film has a thickness of from 3 μm to 25 μm.

(4) A heat-dissipating substrate for LED as described in any one of (1) to (3), wherein a surface of the polyimide film to be bonded to the copper foil or the copper alloy foil, and a surface of the polyimide film to be bonded to the aluminum foil or the aluminum alloy foil comprise a thermo-compression bondable polyimide layer.

(5) A heat-dissipating substrate for LED as described in (4), wherein the polyimide film comprises a heat-resistant polyimide layer, and thermo-compression bondable polyimide layers, which are laminated on both sides of the heat-resistant polyimide layer.

(6) A heat-dissipating substrate for LED as described in any one of (1) to (5), wherein the copper foil or the copper alloy foil has a thickness of from 9 μm to 200 μm, and the aluminum foil or the aluminum alloy foil has a thickness of from 200 μm to 1 mm.

(7) A heat-dissipating substrate for LED as described in any one of (1) to (6), wherein the polyimide film, the copper foil or the copper alloy foil, and the aluminum foil or the aluminum alloy foil are bonded together using a hot-press forming machine.

In this context, the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil (also referred to as “the thermal resistance between the copper foil and the aluminum foil”) is generally equal to the thermal resistance of the polyimide film.

Effect of the Invention

The heat-dissipating substrate for LED according to the present invention comprises a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil, which are laminated on one side and the other side of a thin polyimide film, respectively, and the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil is 1.8° C./W′ or less. There has been no such heat-dissipating substrate for LED, which is thin, and has good bending properties, and has excellent heat-dissipating properties and withstand voltage.

A polyimide film has excellent insulating properties, and therefore a thinner polyimide film provides adequate withstand voltage. Because the polyimide film may be very thin, the heat-dissipating substrate for LED according to the present invention has a reduced total thickness, and has a low thermal resistance and excellent heat-dissipating properties, and also has excellent machine-processability (machinability), and has good bending properties, and therefore is 3-dimensionally processible. The heat-dissipating substrate for LED according to the present invention may be produced in a roll-to-roll process, which is suitable for mass production.

An aluminum foil or an aluminum alloy foil is generally subjected to anodization treatment (alumite treatment) so as to improve the adhesiveness. An aluminum foil or an aluminum alloy foil which has been subjected to anodization treatment, however, has a thick oxide film having a thickness of about 4 μm, for example, which is relatively hard, over the surface. Consequently, a heat-dissipating substrate for LED comprising this type of aluminum foil or aluminum alloy foil may have reduced bending properties. According to the present invention, an aluminum foil or an aluminum alloy foil which has not been subjected to anodization treatment may be preferably employed.

DESCRIPTION OF EMBODIMENTS

The heat-dissipating substrate for LED according to the present invention comprises a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil, which are laminated on one side and the other side of a polyimide film, respectively, and the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil is 1.8° C./W or less. The thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil may be preferably 1.2° C./W or less, more preferably 0.8° C./W or less, particularly preferably 0.6° C./W or less.

The lower limit of the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil may be preferably, but not limited to, 0.1° C./W or more, more preferably 0.15° C./W or more, particularly preferably 0.2° C./W or more, for example.

According to the present invention, the copper foil or the copper alloy foil, and the aluminum foil or the aluminum alloy foil may be preferably laminated directly on the polyimide film without using an adhesive or the like. Accordingly, a surface of the polyimide film to be bonded to the copper foil or the copper alloy foil, and a surface of the polyimide film to be bonded to the aluminum foil or the aluminum alloy foil may be preferably a polyimide layer exhibiting excellent adhesion to the metals, more preferably a thermo-compression bondable polyimide layer exhibiting excellent adhesion to the metals. The polyimide film may be a single-layer polyimide film exhibiting excellent adhesion to the metals, particularly a single-layer thermo-compression bondable polyimide film exhibiting excellent adhesion to the metals, or may be a polyimide film in which two polyimide layers exhibiting excellent adhesion to the metals are laminated on both sides of a heat-resistant polyimide layer, particularly a polyimide film in which two thermo-compression bondable polyimide layers exhibiting excellent adhesion to the metals are laminated on both sides of a heat-resistant polyimide layer. In terms of excellent mechanical properties, preferred is a polyimide film in which two polyimide layers exhibiting excellent adhesion to the metals, more preferably thermo-compression bondable polyimide layers, are laminated on both sides of a heat-resistant polyimide layer.

In the present invention, the polyimide film may be provided in the form of a polyimide film on the completion of the heat-dissipating substrate for LED. The heat-dissipating substrate for LED according to the present invention is not limited to a substrate produced by laminating a polyimide film, a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil together. The heat-dissipating substrate for LED according to the present invention may be produced, for example, by flow-casting or applying a solution of a polyimide precursor, e.g. polyamic acid, on a copper foil or a copper alloy foil; heating the solution to effect imidization, thereby producing a polyimide film; and then laminating an aluminum foil or an aluminum alloy foil onto the polyimide film, for example, by thermo-compression bonding or the like.

The processes for producing the polyimide film and the heat-dissipating substrate for LED will be described later.

The copper foil or the copper alloy foil to be used in the present invention may be preferably, but not limited to, a copper foil, particularly preferably a rolled copper foil or an electrolytic copper foil.

The thickness of the copper foil or the copper alloy foil may be preferably from 9 μm to 200 μm, more preferably from 18 μm to 200 μm. In some embodiments, the copper foil or the copper alloy foil may preferably have a thickness of from 35 μm to 80 μm. The copper foil or the copper alloy foil having a greater thickness may be generally more suitable for high electrical current applications.

The copper foil or the copper alloy foil may preferably have an Rz, which indicates surface roughness, of 3 μm or less, more preferably 2 μm or less, particularly preferably from 0.5 μm to 1.5 μm. When the Rz is small, the surface of the copper foil or the copper alloy foil may be subjected to surface treatment before use.

Examples of the copper foil include rolled copper foils and electrolytic copper foils, and rolled copper alloy foils and electrolytic copper alloy foils. A rolled copper foil may be particularly preferred.

The aluminum foil or the aluminum alloy foil to be used in the present invention is not limited, and the aluminum alloy foil may be made of any alloy of aluminum as the main component and one or more other metals. Examples of the aluminum alloy foil include aluminum alloys containing magnesium as the main additive (Al—Mg alloys) such as JIS 5000 series aluminum alloys, including the JIS 5052 alloy.

In the present invention, an aluminum alloy foil containing at least aluminum and magnesium may be preferred due to its good bending properties.

Although any Al—Mg alloy having any composition may be used in the present invention, the Al—Mg alloy may preferably have a magnesium content of from 1.5 wt % to 5 wt %, more preferably 2 wt % to 3 wt %, due to its excellent strength.

As described above, an aluminum foil or an aluminum alloy foil which has not been subjected to anodization treatment, or an aluminum foil or an aluminum alloy foil which has a rather thin anodized layer (having a thickness of less than 4 μm, for example, further preferably 3 μm or less, particularly preferably 2 μm or less), more preferably an aluminum foil or an aluminum alloy foil which has not been subjected to anodization treatment, may be preferably employed, because a heat-dissipating substrate for LED which has good bending properties and excellent processability may be obtained. When an aluminum foil or an aluminum alloy foil which has not been subjected to anodization treatment, or an aluminum foil or an aluminum alloy foil which has a rather thin anodized layer is employed, excellent flexibility may be achieved and an appearance defect may not be readily caused in the case where the aluminum foil or the aluminum alloy foil is bonded to a polyimide film by thermo-compression bonding.

In addition, although an aluminum foil or an aluminum alloy foil which has been subjected to an alternating-current electrolysis treatment wherein an alkaline electrolyte solution containing a surfactant is used (KO treatment), e.g. KO treated sheets manufactured by Furukawa-Sky Aluminum Corp., may be used in the present invention, an aluminum foil or an aluminum alloy foil which has not been subjected to KO treatment may be preferably employed.

Although the surface of the aluminum foil or the aluminum alloy foil to be laminated onto the polyimide film may have been subjected to anodization treatment (also referred to as “alumite treatment” or “sulfuric-acid anodizing treatment”), or KO treatment, an aluminum foil or an aluminum alloy foil which has not been subjected to anodization treatment nor KO treatment may be preferably employed in terms of heat resistance and bending properties.

The surface of the aluminum foil or the aluminum alloy foil to be bonded to the polyimide film may be preferably treated with an organic solvent to remove oil which is adhered to the surface during the production of the aluminum foil or the aluminum alloy foil.

The thickness of the aluminum foil or the aluminum alloy foil may be preferably from 200 μm to 1 mm, more preferably from 250 μm to 500 μm, particularly preferably from 300 μm to 400 μm. The aluminum foil or the aluminum alloy foil having a smaller thickness may be generally more suitable for bending applications.

According to the present invention, a heat sink may be attached to the aluminum foil or the aluminum alloy foil for enhanced heat dissipation. In the case where a solderable aluminum foil or a solderable aluminum alloy foil, e.g. “SAPlate” manufactured by Toyo Kohan Co., Ltd., is employed, a heat sink may be directly soldered to the aluminum foil or the aluminum alloy foil.

In the present invention, when the polyimide film has a thermo-compression bondable layer as the surface to be bonded to the metal foil (copper foil, aluminum foil), the thickness of the thermo-compression bondable layer may be preferably equal to or greater than the surface roughness (Rzjis) of the surface of the metal foil to be bonded to the polyimide film. When the thickness of the thermo-compression bondable layer is less than the surface roughness (Rzjis) of the metal foil, the heat-dissipating substrate for LED obtained may have peel strength between the polyimide film and the metal foil which varies greatly with position.

There will now be described the polyimide film.

The polyimide film may be, but not limited to, a polyimide film which exhibits excellent adhesiveness to a copper foil and an aluminum foil, and may be preferably thermo-compression bondable, wherein the metal foil such as a copper foil laminated thereon may be removed by etching, and has excellent heat resistance, electrical insulating properties and bending properties. The polyimide film may sufficiently support a metal foil laminated thereon, as necessary, and may not be deteriorated greatly by the action of a developer or remover for removing a photoresist layer which is used for the formation of metal wiring, as necessary.

The polyimide film may be a single-layer film, sheet or plate or a multi-layer film, sheet or plate having two or more layers.

Examples of the polyimide film include, but not limited to, “UPILEX (VT)” (trademark) from Ube Industries, Ltd.

Although the thickness of the polyimide film is not limited, the thinner polyimide film may be more preferred, so long as the polyimide film has adequate electrical insulating properties. The thickness of the polyimide film may be preferably from 3 μm to 50 μm, more preferably from 4 μm to 35 μm, more preferably from 5 μm to 25 μm, more preferably from 7 μm to 15 μm, particularly preferably from 9 μm to 15 μm.

The thickness of the polyimide film may be preferably from 4 μm to 15 μm, more preferably from 7 μm to 12.5 μm, in view of soldering resistance, and may be preferably from 9 μm to 15 μm in view of soldering resistance and heat resistance.

In the present invention, a polyimide film may be used after subjecting at least one surface to surface treatment such as corona discharge treatment, plasma treatment, chemical surface roughening treatment, physical surface roughening treatment, and surface treatment with a surface-treating agent, for example, a silane coupling agent. In the case of a single-layer thermo-compression bondable polyimide film, and in the case where a thermo-compression bondable polyimide layer of a polyimide film is bonded directly to a metal, a surface treatment with a surface-treating agent does not need to be conducted.

Examples of the silane coupling agent to be used for surface treatment of the polyimide film include various silane coupling agents such as amino-functional silane coupling agents and epoxy-functional silane coupling agents, which are most commonly used, and mercapto-functional silane coupling agents, olefin-functional silane coupling agents, and acryl-functional silane coupling agents.

Specific examples of the silane coupling agent include vinyl trimethoxy silane, vinyl tris(2-methoxy ethoxy)silane, vinyl phenyl trimethoxy silane, y-methacryloxypropyl trimethoxy silane, y-glycidoxypropyl trimethoxy silane, 4-glycidyl butyl trimethoxy silane, y-aminopropyl triethoxy silane, N-B-(aminoethyl)-y-aminopropyl trimethoxy silane, N-3-(4-(3-aminopropoxy)butoxy)propyl-3-aminopropyl trimethoxy silane, imidazole silane, triazine silane, and y-mercaptopropyl trimethoxy silane. The surface-treating agent for the polyimide film may be preferably a silane coupling agent such as an aminosilane coupling agent and an epoxysilane coupling agent.

A similar effect may be achieved when the surface of the polyimide film is treated with a titanate-based coupling agent, or a zirconate-based coupling agent, instead of a silane coupling agent.

A surface treatment with a silane coupling agent may be conducted according to any known method.

The term “treatment (surface treatment) with a surface-treating agent such as a silane coupling agent” includes a case in which the surface-treating agent is contained in the surface of the polyimide film without change, and a case in which the surface-treating agent contained in the polyimide film has undergone a change, including chemical change, caused by heat treatment at a temperature of from 320° C. to 550° C., for example, in a polyimide or a polyimide precursor, or an organic solution thereof.

When the polyimide film has inadequate handling properties, e.g. low rigidity of the substrate, a rigid film or plate capable of being peeled therefrom in the later step may be bonded to the back surface of the substrate in use.

In the present invention, the polyimide film may be a multi-layer thermo-compression bondable and/or adhesive polyimide film having two or more layers, which comprises a heat-resistant layer (Sa1) and thermo-compression bondable and/or adhesive layers (Sa2), including a layer of an adhesive agent, on both sides of the heat-resistant layer. Examples of the layer configuration include Sa2/Sa1/Sa2, and Sa2/Sa1/Sa2/Sa1/Sa2. Alternatively, the polyimide film may be comprised of a single thermo-compression bondable layer (Sa2).

The thermo-compression bondable and/or adhesive layer (Sa2) of the polyimide film is used to be bonded to a metal foil. The thermo-compression bondable and/or adhesive layer (Sa2) may be selected from any thermo-compression bondable layer and any adhesive layer.

In the multi-layer thermo-compression bondable and/or adhesive polyimide film having two or more layers, the thicknesses of the heat-resistant layer (Sa1) and the thermo-compression bondable and/or adhesive layer (Sa2) may be appropriately selected. In the present invention, however, the thickness of the thermo-compression bondable and/or adhesive layer (Sa2) which is the outermost layer (surface layer) is preferably equal to or greater than the surface roughness (Rzjis) of the surface of the metal foil to be bonded to the polyimide film, as described above, and it may be preferably 0.5 μm or more, more preferably 1 μm or more, particularly preferably 2 or more. In addition, the thickness of the thermo-compression bondable and/or adhesive layer (Sa2) may be preferably 3 μm or less.

The heat-dissipating substrate for LED of the present invention is characterized in that a copper foil or a copper alloy foil, a polyimide film (polyimide layer), and an aluminum foil or an aluminum alloy foil are laminated together. The present invention should not be limited by the production process.

The heat-dissipating substrate for LED may be, for example, a laminate wherein a copper foil or a copper alloy foil is laminated on one side of a polyimide film and an aluminum foil or an aluminum alloy foil is laminated on the other side of the polyimide film directly, or via an adhesive (thermo-compression bondable material) by the application of heat and/or pressure. Alternatively, the heat-dissipating substrate for LED may be produced by applying solutions of polyimides or polyimide precursors (e.g. solutions of polyamic acids), which are to be converted into thermo-compression bondable polyimide layers, on a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil, respectively, followed by heating, drying and imidization, if necessary; and then laminating a polyimide film onto the obtained laminates by the application of heat and/or pressure. Alternatively, the heat-dissipating substrate for LED may be produced by applying a solution of a polyimide or polyimide precursor (e.g. a solution of a polyamic acid), which are to be converted into a thermo-compression bondable polyimide layer, on a copper foil or a copper alloy foil, followed by heating, drying and imidization, if necessary; and then laminating an aluminum foil or an aluminum alloy foil onto the obtained laminate by the application of heat and/or pressure.

It is preferred that the metal foils (copper foil, aluminum foil) are laminated directly on the polyimide film. When adequate adhesiveness between the surface of the polyimide film and the metal foil may not be achieved by the application of pressure, heat, or combination of both, the metal foil is preferably laminated on the polyimide film via an adhesive.

An adhesive or thermo-compression bondable organic material or resin, or a resin to be converted into a polyimide film may be applied onto the polyimide film and/or the metal foil by any commonly-used method; for example, by a roll coater, a slit coater, or a comma coater.

A heating apparatus, a pressing apparatus, or a pressing and heating apparatus may be used to laminate a metal foil having an adhesive layer or a thermo-compression bondable resin layer thereon and a polyimide film together, or alternatively, to laminate a metal foil and a polyimide film having an adhesive layer or a thermo-compression bondable resin layer thereon together. It is preferred that the heating conditions and the pressing conditions are appropriately selected depending on the type of the material to be used. There are no particular restrictions to the method for laminating a metal foil and a polyimide film, so long as it may be carried out in a continuous mode or in a batch mode. The method may be preferably carried out continuously, using a roll laminator, a double-belt press, or the like.

A polyimide film having excellent heat resistance and electrical insulating properties may be suitably used in the present invention.

The polyimide film may be a single-layer polyimide film, or a multi-layer laminated polyimide film having two or more polyimide layers. There are no particular restrictions to the type of the polyimide.

A polyimide film may be prepared by a known method. A single-layer polyimide film may be prepared, for example, by

(1) flow-casting or applying a solution of a polyamic acid, which is a polyimide precursor, on a support; and then imidizing the polyamic acid; or

(2) flow-casting or applying a solution of a polyimide on a support; and then heating the polyimide solution, if necessary.

A multi-layer polyimide film having two or more polyimide layers may be prepared, for example, by

(3) flow-casting or applying a solution of a polyamic acid, which is a polyimide precursor, on a support; flow-casting or applying a solution of a polyamic acid, which is a polyimide precursor, on the polyamic acid layer, and repeating the procedure successively, to form two or more polyamic acid layers; and then imidizing the polyamic acids;

(4) flow-casting or applying two or more solutions of polyamic acids, which are polyimide precursors, on a support simultaneously, to form two or more polyamic acid layers; and then imidizing the polyamic acids;

(5) flow-casting or applying a solution of a polyimide on a support; flow-casting or applying a solution of a polyimide on the polyimide layer, and repeating the procedure successively, to form two or more polyimide layers; and then heating the polyimide solutions, if necessary;

(6) flow-casting or applying two or more solutions of polyimides on a support simultaneously, to form two or more polyimide layers; and then heating the polyimide solutions, if necessary; or

(7) laminating two or more polyimide films, which are prepared by any one of methods (1) to (6) as described above, directly or via an adhesive.

In addition, a polyimide film may be directly formed on the copper foil or the copper alloy foil, or the aluminum foil or the aluminum alloy foil in the heat-dissipating substrate for LED, which serves as a support.

The polyimide film may be preferably a thermo-compression bondable polyimide film having three or more layers, which comprises a heat-resistant polyimide layer (S1) and thermo-compression bondable polyimide layers (S2) on both sides of the heat-resistant polyimide layer (S1). Examples of the layer configuration of the multi-layer polyimide film include S2/S1/S2, and S2/S1/S2/S1/S2. Alternatively, the polyimide film may be comprised of a single thermo-compression bondable polyimide layer (S2).

In the thermo-compression bondable polyimide film, the thicknesses of the heat-resistant polyimide layer (S1) and the thermo-compression bondable polyimide layer (S2) may be appropriately selected. In the present invention, however, the thickness of the thermo-compression bondable polyimide layer (S2) which is the outermost layer (surface layer) is preferably equal to or greater than the surface roughness (Rzjis) of the surface of the metal foil to be bonded to the polyimide film, as described above. In the case of a polyimide film having thermo-compression bondable polyimide layers (S2) on both sides of a heat-resistant polyimide layer (S1), the thermo-compression bondable polyimide layer (S2) which is the outermost layer should have a thickness sufficient to achieve adequate adhesiveness to the copper foil or the aluminum foil by thermo-compression bonding. The thickness of the thermo-compression bondable polyimide layer (S2) may be preferably 0.5 μm to 10 μm, more preferably 1 μm to 7 μm, further preferably 2 μm to 5 μm. In addition, the thickness of the thermo-compression bondable polyimide layer (S2) may be preferably 3 μm or less.

The curling of the polyimide film may be reduced when two thermo-compression bondable polyimide layers (S2) which are substantially equal in thickness are laminated on both sides of the heat-resistant polyimide layer (S1).

A heat-resistant polyimide for the heat-resistant polyimide layer (S1) in the thermo-compression bondable polyimide film may have at least one of the following features (1) to (4), particularly at least two of the following features (1) to (4) [combinations of (1) and (2), (1) and (3), (2) and (3), etc.], and particularly preferably have all of the following features.

(1) In the form of a separate polyimide film, the glass transition temperature is 300° C. or higher, preferably 330° C. or higher, and more preferably, a glass transition temperature is undetectable.

(2) In the form of a separate polyimide film, the coefficient of thermal expansion (50° C. to 200° C.) (MD) is close to a coefficient of thermal expansion of a metal foil such as a copper foil to be laminated on the polyimide film. Specifically, a coefficient of thermal expansion of the polyimide film is preferably 5×10⁻⁶ cm/cm/° C. to 28×10⁻⁶ cm/cm/° C., more preferably 9×10⁻⁶ cm/cm/° C. to 20×10⁻⁶ cm/cm/° C., further preferably 12×10⁻⁶ cm/cm/° C. to 18×10⁻⁶ cm/cm/° C.

(3) In the form of a separate polyimide film, the tensile modulus of elasticity (MD, ASTM-D882) is 300 kg/mm² or more, preferably 500 kg/mm² or more, further preferably 700 kg/mm² or more.

(4) The heat shrinkage is preferably 0.05% or less.

A polyimide which is prepared from an acid component comprising at least one selected from the group consisting of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA), pyromellitic dianhydride (PMDA) and 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) as the main component, and a diamine component comprising at least one selected from the group consisting of p-phenylenediamine (PPD) and 4,4′-diaminodiphenyl ether (DADE) as the main component may be used for the heat-resistant polyimide layer (S1). A part of, or all of 4,4′-diaminodiphenyl ether (DADE) may be replaced with 3,4′-diaminodiphenyl ether (DADE).

The following polyimides, for example, may be suitably used for the heat-resistant polyimide layer (S1).

(1) a polyimide prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA), and p-phenylenediamine (PPD) and optionally 4,4′-diaminodiphenyl ether (DADE). In this polyimide, a ratio of PPD/DADE (molar ratio) is preferably 100/0 to 85/15.

(2) a polyimide prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) and pyromellitic dianhydride (PMDA), and p-phenylenediamine (PPD) and 4,4′-diaminodiphenyl ether (DADE). In this polyimide, a ratio of BPDA/PMDA (molar ratio) is preferably 15/85 to 85/15, and a ratio of PPD/DADE (molar ratio) is preferably 90/10 to 10/90.

(3) a polyimide prepared from pyromellitic dianhydride (PMDA), and p-phenylenediamine (PPD) and 4,4′-diaminodiphenyl ether (DADE). In this polyimide, a ratio of DADE/PPD (molar ratio) is preferably 90/10 to 10/90.

(4) a polyimide prepared from 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA), and p-phenylenediamine (PPD) and 4,4′-diaminodiphenyl ether (DADE). In this polyimide, a ratio of BTDA/PMDA (molar ratio) is preferably 20/80 to 90/10, and a ratio of PPD/DADE (molar ratio) is preferably 30/70 to 90/10.

Other tetracarboxylic dianhydrides and other diamines may be used, so long as the properties of the heat-resistant polyimide would not be imp aired.

A heat-resistant polyimide for the heat-resistant polyimide layer (S1 layer) may be synthesized by random polymerization or block polymerization of an acid component and a diamine component, both of which have the final compositions in the above-mentioned range. A heat-resistant polyimide for the heat-resistant polyimide layer (S1 layer) may also be synthesized by synthesizing two polyamic acids, and then mixing these polyamic acid solutions under the reaction conditions to form a homogeneous solution.

The heat-resistant polyimide may be prepared as described below. First, a polyamic acid solution is prepared by reacting substantially equimolar amounts of a diamine component and an acid component (tetracarboxylic dianhydride), both of which are described above, in an organic solvent. In the polyamic acid solution, imidization may partially proceed, so long as the polyamic acid solution remains homogeneous. Subsequently, the polyamic acid solution is used as a dope, and a thin film of the dope is formed, and then heated to evaporate and remove the solvent from the thin film and to imidize the polyamic acid, thereby preparing a heat-resistant polyimide.

According to the present invention, a thin film of a dope for a thermo-compression bondable polyimide may be laminated onto a thin film of a dope for a heat-resistant polyimide, followed by simultaneous imidization of these dopes. According to the present invention, a dope for a heat-resistant polyimide and a dope for a thermo-compression bondable polyimide may be co-extruded to form a laminate of a thin film of the dope for the heat-resistant polyimide and a thin film of the dope for the thermo-compression bondable polyimide, followed by simultaneous imidization of these dopes. The processes will be described later.

A thermo-compression bondable polyimide for the thermo-compression bondable polyimide layer (S2) is (1) a polyimide which may be thermo-compression bondable to a metal foil. The thermo-compression bondable polyimide may be preferably laminated to a metal foil at a temperature of from the glass transition temperature of the thermo-compression bondable polyimide to 400° C.

In addition, the thermo-compression bondable polyimide for the thermo-compression bondable polyimide layer (S2) may preferably have at least one of the following features (2) to (5).

(2) The peel strength between the metal foil and the polyimide (S2) is 0.7 N/mm or more, and the retention rate of the peel strength after heat treatment at 150° C. for 168 hours is 90% or more, further preferably 95% or more, particularly preferably 100% or more.

(3) The glass transition temperature is within a range of from 130° C. to 330° C.

(4) The tensile modulus of elasticity is within a range of from 100 kg/mm² to 700 kg/mm².

(5) The coefficient of thermal expansion (50° C. to 200° C.) (MD) is within a range of from 13×10⁻⁶ cm/cm/° C. to 30×10⁻⁶ cm/cm/° C.

Various known thermoplastic polyimides may be selected as the thermo-compression bondable polyimide for the thermo-compression bondable polyimide layer (S2). The thermo-compression bondable polyimide to be used may be, for example, a polyimide which is prepared from

an acid component comprising at least one selected from the group consisting of 2, 3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA), pyromellitic dianhydride (PMDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 3,3%4,4′-diphenyl sulfone tetracarboxylic dianhydride, 4,4′-oxydiphthalic dianhydride (ODPA), p-phenylene bis(trimellitic acid monoester anhydride), and 3,3′,4,4′-ethylene glycol dibenzoate tetracarboxylic dianhydride, for example, preferably an acid component comprising them as the main component, and

a diamine component comprising at least one diamine having at least three benzene rings in the main chain, which is selected from the group consisting of 1, 4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, bis[4-(4-aminophenoxy)phenyl] sulfone, and bis[4-(3-aminophenoxy)phenyl] sulfone, for example, preferably a diamine component comprising them as the main component, which may comprise a diamine having one or two benzene rings in the main chain, as necessary.

A polyimide which is prepared from an acid component selected from the group consisting of 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA), pyromellitic dianhydride (PMDA) and 3,3′,4, 4′-benzophenone tetracarboxylic dianhydride (BTDA), and a diamine component selected from the group consisting of 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene and 2,2-bis[4-(4-aminophenoxy)phenyl]propane may be suitably used as the thermo-compression bondable polyimide. This thermo-compression bondable polyimide may comprise a diamine having one or two benzene rings in the main chain, and/or other diamine components, and/or other acid components, as necessary.

A particularly preferable thermo-compression bondable polyimide may be a polyimide prepared from a diamine component comprising not less than 80 mol % of 1,3-bis(4-aminophenoxy benzene) (hereinafter, sometimes abbreviated as “TPER”), and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) and/or 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA). In this polyimide, a ratio of s-BPDA/a-BPDA (molar ratio) is preferably 100/0 to 5/95. Other tetracarboxylic dianhydrides such as 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride and 2,3,6,7-naphthalene tetracarboxylic dianhydride may be used, so long as the properties of the thermo-compression bondable polyimide would not be impaired.

The thermo-compression bondable polyimide may be prepared as described below. First, a polyamic acid solution is prepared by reacting a diamine component and an acid component, both of which are described above, and optionally another tetracarboxylic dianhydride and another diamine in an organic solvent at a temperature of about 100° C. or lower, particularly from 20° C. to 60° C. Subsequently, the polyamic acid solution is used as a dope, and a thin film of the dope is formed, and then heated to evaporate and remove the solvent from the thin film and to imidize the polyamic acid, thereby preparing a thermo-compression bondable polyimide.

A solution of a thermo-compression bondable polyimide in an organic solvent may be prepared as described below. First, a polyamic acid solution, which is prepared as described above, is heated at a temperature of from 150° C. to 250° C. to effect imidization. Alternatively, an imidizing agent is added to the polyamic acid solution, and then the polyamic acid solution is reacted at a temperature of 150° C. or lower, particularly from 15° C. to 50° C., to effect imidization. Subsequently, the thermo-compression bondable polyimide powder is obtained by evaporating the solvent, or alternatively, by precipitating the thermo-compression bondable polyimide in a poor solvent. And then, the powder is dissolved in an organic solvent, thereby preparing a solution of the thermo-compression bondable polyimide in the organic solvent.

In the preparation of the thermo-compression bondable polyimide, a ratio of a diamine (as the number of moles of amino groups) to an acid anhydride (as the total number of moles of acid anhydride groups in a tetracarboxylic dianhydride and a dicarboxylic anhydride) in an organic solvent as described above may be preferably 0.95 to 1.0, particularly preferably 0.98 to 1.0, further preferably 0.99 to 1.0. When a dicarboxylic anhydride is used for the reaction, the dicarboxylic anhydride may be used in a ratio of 0.05 or less relative to the number of moles of acid anhydride groups in a tetracarboxylic dianhydride.

When a polyamic acid obtained in the preparation of the thermo-compression bondable polyimide has a low molecular weight, adhesion strength of a laminate of the polyimide film and a metal foil, i.e. a heat-dissipating substrate for LED of the present invention, may be reduced.

For the purpose of preventing gelation of the polyamic acid, a phosphorus-containing stabilizer such as triphenyl phosphite and triphenyl phosphate may be added to the solution in an amount of 0.01 wt % to 1 wt % based on the solid (polymer) content during the polymerization of the polyamic acid.

For the purpose of accelerating imidization, a basic organic compound may be added to the dope. For example, imidazole, 2-methylimidazole, 1,2-dimethylimidazole, 2-phenylimidazole, benzimidazole, isoquinoline, a substituted pyridine, and the like may be added to the dope in an amount of 0.05 wt % to 10 wt %, particularly 0.1 wt % to 2 wt %, based on the polyamic acid. These compounds may be used to avoid insufficient imidization, which may occur in the formation of the polyimide film at a relatively low temperature.

In addition, for the purpose of stabilizing adhesiveness, an organic aluminum compound, an inorganic aluminum compound, or an organic tin compound may be added to the polyamic acid solution for the thermo-compression bondable polyimide. For example, aluminum hydroxide, aluminum triacetylacetonate, and the like may be added to the polyamic acid solution in an amount of 1 ppm or more, particularly 1 ppm to 1,000 ppm in terms of aluminum metal, based on the polyamic acid.

Examples of the organic solvent to be used for the preparation of the polyamic acid from an acid component and a diamine component both for the heat-resistant polyimide and for the thermo-compression bondable polyimide include N-methyl-2-pyrrolidone, N, N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, dimethyl sulfoxide, hexamethyl phosphoramide, N-methylcaprolactam, and cresol. These organic solvents may be used alone or in combination of two or more.

In the preparations of the heat-resistant polyimide and the thermo-compression bondable polyimide, a dicarboxylic anhydride such as phthalic anhydride and substituted derivative thereof, hexahydrophthalic anhydride and substituted derivative thereof, and succinic anhydride and substituted derivative thereof, particularly phthalic anhydride, may be used for end-capping the amine terminal.

A thermo-compression bondable polyimide film having a thermo-compression bondable polyimide layer (S2) on one side or both sides of a heat-resistant polyimide layer (S1) may be suitably prepared by

(i) a method wherein a multi-layer polyimide film is formed by flow-casting a dope for the heat-resistant polyimide (S1) and a dope for the thermo-compression bondable polyimide (S2) on a support in the form of a thin film laminate by coextrusion-flow casting method (sometimes referred to as “coextrusion method”), and then drying and imidizing the dopes; or

(ii) a method wherein a multi-layer polyimide film is formed by applying a dope for the thermo-compression bondable polyimide (S2) onto one side or both sides of a self-supporting film (gel film), which is prepared by flow-casting a dope for the heat-resistant polyimide (S1) on a support and drying the dope, and then drying and imidizing the dopes.

A coextrusion method described in JP-A-1991-180343 (JP-B-1995-102661) may be applied.

There will be described an example of a process for producing a polyimide film having three layers, both sides of which are thermo-compression bondable.

A polyamic acid solution for the heat-resistant polyimide (51) and a polyamic acid solution for the thermo-compression bondable polyimide (S2) are fed to a three-layer extrusion die and flow-cast on a support such as a stainless specular surface and a stainless belt surface by a three-layer coextrusion method such that the thickness of the heat-resistant polyimide layer (S1 layer) is within a range of from 4 μm to 45 μm and the total thickness of the thermo-compression bondable polyimide layers (S2 layers) on both sides of the heat-resistant polyimide layer (S1 layer) is within a range of from 1 μm to 20 μm. And then, the flow-cast film is dried at a temperature of from 100° C. to 200° C., thereby producing a polyimide film (A) as a self-supporting film in a semi-cured state or in a dried state which is an earlier stage.

In the preparation of the thermo-compression bondable polyimide film, when the flow-cast film is heated at a high temperature higher than 200° C., adhesiveness may be reduced, for example. The term “in a semi-cured state or in a state which is an earlier stage” means that the film is in a self-supporting state by heating and/or chemical imidization.

The polyimide film (A) which is a self-supporting film may preferably contain the solvent and water, which has formed by the reaction, in an amount of about 25 wt % to about 60 wt %, particularly preferably 30 wt % to 50 wt %. The self-supporting film may be preferably heated up to the temperature for drying/imidization in a relatively short period of time; for example, the self-supporting film may be preferably heated at a temperature-increasing rate of 10° C./min or more.

When higher tension is applied to the self-supporting film during drying/imidization, the polyimide film (A) finally obtained may have a lower coefficient of thermal expansion.

Subsequent to the drying step for the preparation of the self-supporting film as described above, successively or non-successively, the self-supporting film is dried and heated preferably for about 1 min to 100 min, particularly preferably 1 min to 10 min, at a high temperature higher than the drying temperature as described above, preferably at a temperature of from 200° C. to 550° C., particularly preferably from 300° C. to 500° C., for example, while fixing at least one pair of edges of the self-supporting film with a fixing device which is movable therewith, and the like. A polyimide film, both sides of which are thermo-compression bondable, is formed by sufficiently removing the solvent and the like from the self-supporting film and fully imidizing the polymer constituting the film such that the polyimide film finally obtained preferably has a volatile content (the amount of volatile component such as the organic solvent and water, which has formed by the reaction) of 1 wt % or less.

A preferable device to fix a self-supporting film as described above may be, for example, a device wherein a pair of belts or chains having many equally-spaced pins or grippers, for example, are disposed along both edges of a self-supporting film, which is fed continuously or intermittently, in the length direction, and move with the self-supporting film continuously or intermittently, while fixing the film. In addition, a device to fix a self-supporting film as described above may be a device capable of stretching and/or shrinking the film in the width direction or in the length direction during heat treatment at an appropriate stretch or shrink ratio (particularly preferably about 0.5% to about 5%).

The polyimide film, both sides of which are thermo-compression bondable, prepared in the process as described above may be further heated at a temperature of from 100° C. to 400° C. preferably for 0.1 min to 30 min under no tension or a low tension, preferably under a low tension of 4 N or lower, particularly preferably 3 N or lower, to provide a polyimide film having improved dimensional stability, in particular, both sides of which are thermo-compression bondable.

The long polyimide film, both sides of which are thermo-compression bondable, thus prepared may be wound into a roll by an appropriate known method.

The surface of the polyimide film in which the flow-cast dope was in contact with a support was taken as Side B, while the surface of the polyimide film in which the flow-cast dope was not in contact with a support (air side) was taken as Side A.

There will be described an example of a process for producing a heat-dissipating substrate for LED, which comprises a polyimide film having a heat-resistant polyimide layer (S1) and thermo-compression bondable polyimide layers (S2) on both sides of the heat-resistant polyimide layer (S1) as described above, in particular.

The heat-dissipating substrate for LED may be prepared, for example, by laminating metal foils, i.e. a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil, onto both sides of the heat-resistant polyimide (51) as described above, directly or via an adhesive.

The heat-dissipating substrate for LED may be preferably prepared with a polyimide film having thermo-compression bondable polyimide layers (S2) on both sides as described above, and prepared by directly laminating metal foils, i.e. a copper foil or a copper alloy foil, and an aluminum foil or an aluminum alloy foil, onto the thermo-compression bondable polyimide layers (S2).

In view of the peel strength, the heat-dissipating substrate for LED may be more preferably prepared with a polyimide film having thermo-compression bondable polyimide layers (S2) on both sides as described above, and prepared by directly laminating a copper foil or a copper alloy foil onto the film surface (Side A) of the thermo-compression bondable polyimide layer (S2) and directly laminating an aluminum foil or an aluminum alloy foil onto the film surface (Side B) of the thermo-compression bondable polyimide layer (S2).

Examples of the process for producing the heat-dissipating substrate for LED, in which metal foils are laminated on both sides of a thermo-compression bondable polyimide film, include the following processes.

1) A long metal foil (a copper foil or a copper alloy foil), a long thermo-compression bondable polyimide film, and a long metal foil (an aluminum foil or an aluminum alloy foil) are laminated in this order, and fed to a thermo-compression bonding apparatus (a heating and pressing apparatus). The metal foils and the thermo-compression bondable polyimide film may be preferably pre-heated at a temperature of from about 150° C. to about 250° C., particularly preferably at a temperature higher than 150° C. but not higher than 250° C., for about 2 sec to about 120 sec, using a pre-heating apparatus such as a hot-air blower and an infrared heater, preferably in-line just before introducing into the thermo-compression bonding apparatus.

2) A three-layer laminate (metal foil/polyimide/metal foil) is subjected to thermo-compression bonding under pressure at a temperature equal to or higher than the temperature higher by 20° C. than the glass transition temperature of the polyimide (S2), but not higher than 400° C., particularly preferably at a temperature equal to or higher than the temperature higher by 30° C. than the glass transition temperature, but not higher than 400° C., in the thermo-compression bonding zone, using a pair of press rolls or a double-belt press.

3) In the case of a double-belt press, in particular, subsequent to the thermo-compression bonding, the laminate is cooled under pressure, preferably to a temperature equal to or lower than the temperature lower by 20° C. than the glass transition temperature of the polyimide (S2), particularly preferably to a temperature equal to or lower than the temperature lower by 30° C. than the glass transition temperature, in the cooling zone, to laminate the metal foils onto both sides of the polyimide film, and then the laminate is wound into a roll.

Thus a heat-dissipating substrate for LED may be prepared in the form of a roll.

In the processes, the pre-heating prior to the thermo-compression bonding allows the prevention of defects in appearance of the laminate obtained after the thermo-compression bonding, which is due to foaming caused by water and the like in the polyimide, and the prevention of foaming during the immersion in a solder bath in the formation of an electronic circuit, which results in the prevention of the reduction in product yield. It is impractical to place a whole thermo-compression bonding apparatus in a heating furnace, because the thermo-compression bonding apparatus to be used is substantially limited to a compact one, leading to the restriction of the shape of the heat-dissipating substrate for LED. If out-line pre-heating is conducted, the polyimide may absorb water before lamination, and therefore it may be difficult to prevent defects in appearance of the laminate obtained after the thermo-compression bonding due to foaming, and reduction in solder heat resistance.

A double-belt press is capable of conducting high-temperature heating/cooling under pressure, and a hydraulic-press type using a heat medium may be preferred.

When metal foils are laminated onto a polyimide film, both sides of which are thermo-compression bondable, by thermo-compression bonding/cooling under pressure with a double-belt press, the take-off speed may be preferably 1 m/min or higher, and there may be provided a long and wide metal foil laminated polyimide film (heat-dissipating substrate for LED) having a width of about 400 mm or more, particularly about 500 mm or more, and having a high adhesive strength (i.e. high peel strength between the metal foil and the polyimide layer) and such good appearance that substantially no wrinkles are observed in the metal foil surface.

It is preferred that one or more combinations of a thermo-compression bondable polyimide film and a metal foil are fed, together with protective films (i.e. two protective films), each of which is placed between the outermost layer and the belt, and laminated by thermo-compression bonding/cooling under pressure so as to mass-produce a heat-dissipating substrate for LED having good appearance. Any protective film may be used, irrespective of the material, so long as it is non-thermo-compression bondable and has excellent surface smoothness. Preferable examples of the protective film include a metal foil, specifically a copper foil, a stainless steel foil or an aluminum foil, and a highly heat-resistant polyimide film (UPILEX S from Ube Industries, Ltd.) which have a thickness of about 5 μm to about 125 μm.

According to any known method, a metal wiring may be formed on the heat-dissipating substrate for LED thus obtained by partially removing the copper foil or the copper alloy foil by etching, and a LED chip may be mounted on the metal-wiring side. The heat-dissipating substrate for LED according to the present invention has excellent heat-dissipating properties, and therefore a rise in temperature of the LED and a reduction in the luminous efficiency of the LED may be reduced even when many LEDs are mounted on the substrate for use in an LED lighting device or an LED back light.

EXAMPLES

The present invention will be described in more detail below with reference to the Examples. However, the present invention is not limited to these Examples.

(Evaluation Methods)

(1) Peel strength: The peel strength between the polyimide film and the aluminum foil is measured in accordance with JIS C5016 under the conditions of 90° and a width of 5 mm. The measurement sample is a laminate of polyimide/aluminum foil which is prepared by peeling the copper foil from the heat-dissipating substrate for LED. The measurements of the peel strengths are carried out at three points, that is, at the initial point without any treatment, after moist heat treatment (after treatment at 85° C. and 85% Rh for 1000 hr), and after solder heat treatment at 260° C. for 30 sec.

(2) Solder heat resistance: The solder heat resistance is evaluated in accordance with JIS C6481. The measurement sample is a laminate of polyimide/aluminum foil which is prepared by removing the copper foil by etching from the heat-dissipating substrate for LED. As the evaluation of the solder heat resistance, the presence or absence of foaming in the polyimide film of the laminate is visually observed after solder heat treatment at 250° C. or 270° C. for 30 sec.

∘: No foaming is observed.

x: Foaming is observed.

(3) Bending processability: The measurement sample is a laminate of copper foil/polyimide/aluminum foil. The laminate is bent such that the laminate has an outside diameter of about 1.0 mm and an inside diameter of about 0.6 mm. Subsequently, the bent part is returned to the initial state, and the presence or absence of cracks in the bent part of the aluminum foil is visually observed. The laminate is bent in two ways, that is, bent with the copper foil outward, and bent with the copper foil inward.

∘: No crack is observed.

x: A crack is observed.

Reference Example 1 Preparation of a Dope for a Heat-Resistant polyimide S1

To N-methyl-2-pyrrolidone were added p-phenylenediamine (PPD) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) in a molar ratio of 1000:998 such that a monomer concentration was 18% (by weight, the same shall apply hereinafter). And then, the mixture was reacted at 50° C. for 3 hours. The polyamic acid solution thus obtained had a solution viscosity of about 1680 poise at 25° C.

Reference Example 2 Preparation of a dope for a thermo-compression bondable polyimide S2

To N-methyl-2-pyrrolidone were added 1,3-bis(4-aminophenoxy) benzene (TPE-R), 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) in a molar ratio of 1000:200:800 such that a monomer concentration was 18%. Subsequently, triphenyl phosphate was added thereto in an amount of 0.5% by weight relative to the monomers, and then the mixture was reacted at 40° C. for 3 hours. The polyamic acid solution thus obtained had a solution viscosity of about 1680 poise at 25° C.

Reference Examples 3, 4 Preparation of Thermo-Compression Bondable Multi-Layer Polyimide Films A1, A2)

Using a film-forming machine equipped with a three-layer extrusion die (multi-manifold type die), the polyamic acid solutions prepared in Reference Examples 1 and 2 were flow-cast on a metal support, with varying a thickness of the three-layer extrusion die, and then continuously dried under hot air at 140° C. and peeled from the support, to form a self-supporting film. The self-supporting film, which was peeled from the support, was gradually heated from 150° C. to 450° C. in a heating furnace for solvent removal and imidization. Thus two long three-layer polyimide films with different thicknesses were prepared and wound into a roll.

The properties of the three-layer polyimide films (layer configuration: S2/S1/S2) thus obtained were evaluated, and the results are as follows.

(Thermo-Compression Bondable Multi-Layer Polyimide Film A1)

-   -   Thickness configuration: 2.5 μm/7.5 μm/2.5 μm (total: 12.5 μm)     -   Glass transition temperature of the S2 layer: 240° C.     -   Glass transition temperature of the S1 layer:         -   Tg was not clearly observed at a temperature of 300° C. or             higher.     -   Coefficient of thermal expansion (50° C. to 200° C.):         -   MD 19 ppm/° C., TD 18 ppm/° C.,     -   Mechanical properties (Test method: ASTM-D882)         -   1) Tensile strength: MD, TD 520 MPa (each)         -   2) Elongation percentage: MD, TD 90% (each)         -   3) Tensile modulus of elasticity: MD, TD 7200 MPa (each)     -   Electrical properties (Test method: ASTM-D149)         -   1) Dielectric breakdown voltage: 4.9 kV

(Thermo-compression bondable multi-layer polyimide film A2)

-   -   Thickness configuration: 4 μm/17 μm/4 μm (total: 25 μm)     -   Glass transition temperature of the S2 layer: 240° C.     -   Glass transition temperature of the S1 layer:         -   Tg was not clearly observed at a temperature of 300° C. or             higher.     -   Coefficient of thermal expansion (50° C. to 200° C.):         -   MD 19 ppm/° C., TD 18 ppm/° C.,     -   Mechanical properties (Test method: ASTM-D882)         -   1) Tensile strength: MD, TD 520 MPa (each)         -   2) Elongation percentage: MD, TD 100% (each)         -   3) Tensile modulus of elasticity: MD, TD 7200 MPa (each)     -   Electrical properties (Test method: ASTM-D149)         -   1) Dielectric breakdown voltage: 7.1 kV

The thermo-compression bondable multi-layer polyimide films A1 and A2 had a thickness of 12.5 μm and 25 μm, respectively, and were thinner than an epoxy resin film used in a conventional and commonest heat-dissipating substrate for LED (thickness: 1.2 mm) and had an electrical insulating properties equivalent to the epoxy resin film.

In the following Examples, a copper foil was laminated on the Side A of the polyimide film, and an aluminum foil was laminated on the Side B of the polyimide film.

The aluminum foil was used after the treatment with an organic solvent to remove oil adhered to the surface.

Examples 1 to 3 Production of Heat-Dissipating Substrate for LED

The three-layer polyimide film A2 was pre-heated in-line by hot air at 200° C. for 30 sec immediately before being fed to a double-belt press. An electrolytic copper foil (thickness: 18 Rz: 0.6 μm) was laminated on one side (Side A) of the polyimide film A2, and an untreated or surface-treated Al—Mg alloy foil (A5052-H34, manufactured by Furukawa-Sky Aluminum Corp., thickness: 300 μm) as shown in Table 1 was laminated on the other side (Side B) of the polyimide film A2. Subsequently, the laminate was fed into a heating zone (the highest heating temperature: 330° C.) and then into a cooling zone (the lowest cooling temperature: 180° C.) to perform continuous thermo-compression bonding and cooling in which the thermo-compression bonding pressure was 3.9 MPa, and the thermo-compression bonding time was 2 min. Thus a heat-dissipating substrate for LED (width: 540 mm, length: 30 m) was produced and wound into a roll. The peel strength, solder heat resistance, and bending processability of the heat-dissipating substrate for LED thus obtained were determined. The results are shown in Table 2. The evaluation of the solder heat resistance was carried out at a temperature of 250° C.

TABLE 1 Surface treatment of Aluminum foil to be laminated onto Side B of Polyimide film Example 1 Untreated Example 2 Sulfuric-acid anodizing treatment Example 3 KO treatment (treated layer: about 1.5 μm)

TABLE 2 Peel strength (N/mm) (between A1 foil/Polyimide) Solder heat Bending processability After moist After solder resistance Bending with Bending with Initial heat treatment heat treatment (250° C.) copper foil outward copper foil inward Example 1 2.7  2.9 3.1 ∘ ∘ ∘ Example 2 3.1 4< 3.6 x ∘ x Example 3 4<  4< 4<  x ∘ x

Examples 4 to 5 Production of Heat-Dissipating Substrate for LED

The three-layer polyimide film as shown in Table 3 was pre-heated in-line by hot air at 200° C. for 30 sec immediately before being fed to a double-belt press. A rolled copper foil (thickness: 35 μM, Roughened grade, Rz: 1.2 μm) was laminated on one side (Side A) of the polyimide film, and an untreated Al—Mg alloy foil (A5052-H34, manufactured by Furukawa-Sky Aluminum Corp., thickness: 300 μm) was laminated on the other side (Side B) of the polyimide film. Subsequently, the laminate was fed into a heating zone (the highest heating temperature: 330° C.) and then into a cooling zone (the lowest cooling temperature: 180° C.) to perform continuous thermo-compression bonding and cooling in which the thermo-compression bonding pressure was 3.9 MPa, and the thermo-compression bonding time was 2 min. Thus a heat-dissipating substrate for LED (width: 540 mm, length: 30 m) was produced and wound into a roll. The peel strength, solder heat resistance, and bending processability of the heat-dissipating substrate for LED thus obtained were determined. The results are shown in Table 3. The evaluation of the solder heat resistance was carried out at a temperature of 250° C. and 270° C.

TABLE 3 Peel strength (N/mm) Bending processability (between A1 foil/Polyimide) Solder heat Bending with Bending with Polyimide After moist After solder resistance copper foil copper foil film Initial heat treatment heat treatment 250° C. 270° C. outward inward Example 4 A1 1.7 1.6 1.7 ∘ ∘ ∘ ∘ Example 5 A2 3.1 3.3 3.5 ∘ x ∘ ∘

(Evaluation of Thermal Resistance)

The thermal resistances of the heat-dissipating substrates for LED produced in Examples 4 and 5 were evaluated according to the following method.

First, the heat-dissipating substrate for LED produced was cut into a size of 1 cm×1.5 cm. Subsequently, a thermally conductive grease was thinly applied to a water cooling plate made of copper and having a size (area) of 10 cm×10 cm, and to the aluminum foil of the heat-dissipating substrate for LED, and then both were bonded together. Subsequently, a thermally conductive grease was thinly applied to the copper foil of the heat-dissipating substrate for LED, and to a transistor (2SC3258) having a side-size of 1 cm×1.5 cm, and then both were bonded together. Grooves for insertion of fine thermocouples were provided previously in the surface of the transistor, and in the surface of the water cooling plate in order to measure the temperature (Th) of the surface of the transistor at the interface between the transistor and the heat-dissipating substrate for LED, and the temperature (Tl) of the surface of the water cooling plate at the interface between the water cooling plate and the copper foil of the heat-dissipating substrate for LED.

As described above, using the heat-dissipating substrate for LED produced, the laminate of transistor/copper foil/polyimide film/Al—Mg alloy foil/water cooling plate, and the fine thermocouples were set.

And then, an electrical power (P) of from 5 W to 35 W was applied to the transistor, and Th and Tl were measured after the thermocouples indicated constant values.

The thermal resistance (Rth) was calculated from the following formula.

Rth=(Th−Tl)/P−2×Rg

wherein Rg represents the thermal resistance of one layer of the thermally conductive grease (0.35° C./W).

The thermal resistance of the heat-dissipating substrate for LED produced in Example 4 was determined as described above, and was calculated to be 0.22° C./W. The heat-dissipating substrate for LED had a very good thermal resistance. Moreover, the thermal resistance of the heat-dissipating substrate for LED produced in Example 5 was determined, and was calculated to be 0.58° C./W. The heat-dissipating substrate for LED had a good thermal resistance.

Example 6 Production of Heat-Dissipating Substrate for LED

The three-layer polyimide film A1 was pre-heated in-line by hot air at 200° C. for 30 sec immediately before being fed to a double-belt press. An electrolytic copper foil (thickness: 18 μm) was laminated on one side of the polyimide film A1, and an Al—Mg alloy foil (JIS 5052-H32 (A5052-H32), manufactured by Furukawa-Sky Aluminum Corp., thickness: 300 μm) was laminated on the other side of the polyimide film A1. Subsequently, the laminate was fed into a heating zone (the highest heating temperature: 330° C.) and then into a cooling zone (the lowest cooling temperature: 180° C.) to perform continuous thermo-compression bonding and cooling in which the thermo-compression bonding pressure was 3.9 MPa, and the thermo-compression bonding time was 2 min. Thus a heat-dissipating substrate for LED (width: 540 mm, length: 30 m) was produced and wound into a roll. The heat-dissipating substrate for LED thus obtained was a double-sided metal foil laminate of copper foil/polyimide film A1/Al—Mg alloy foil, and was bendable and had good bending properties, whereas a conventional and commonest heat-dissipating substrate for LED was not bendable.

(Evaluation of Thermal Resistance)

The thermal resistance of the heat-dissipating substrate for LED produced in Example 6 was evaluated in the same manner as in Example 4. The thermal resistance was 0.24° C./W, and the heat-dissipating substrate for LED had a good thermal resistance.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, there may be provided a heat-dissipating substrate for LED, which is thin, and has excellent heat-dissipating properties and withstand voltage, and also has good bending properties and, for example, may be bent inwardly and may be bent outwardly, and therefore is 3-dimensionally processible. 

1-7. (canceled)
 8. A heat-dissipating substrate for LED, comprising a polyimide film, a copper foil or a copper alloy foil, which is laminated on one side of the polyimide film, and an aluminum foil or an aluminum alloy foil, which is laminated on the other side of the polyimide film; wherein the thermal resistance between the surface of the copper foil or the copper alloy foil and the surface of the aluminum foil or the aluminum alloy foil is 1.8° C./W or less.
 9. A heat-dissipating substrate for LED as claimed in claim 8, wherein the aluminum foil or the aluminum alloy foil is not subjected to anodization treatment (alumite treatment).
 10. A heat-dissipating substrate for LED as claimed in claim 8, wherein the polyimide film has a thickness of from 3 μm to 25 μm.
 11. A heat-dissipating substrate for LED as claimed in claim 8, wherein a surface of the polyimide film to be bonded to the copper foil or the copper alloy foil, and a surface of the polyimide film to be bonded to the aluminum foil or the aluminum alloy foil comprise a thermo-compression bondable polyimide layer.
 12. A heat-dissipating substrate for LED as claimed in claim 11, wherein the polyimide film comprises a heat-resistant polyimide layer, and thermo-compression bondable polyimide layers, which are laminated on both sides of the heat-resistant polyimide layer.
 13. A heat-dissipating substrate for LED as claimed in claim 8, wherein the copper foil or the copper alloy foil has a thickness of from 9 μm to 200 μm, and the aluminum foil or the aluminum alloy foil has a thickness of from 200 μm to 1 mm.
 14. A heat-dissipating substrate for LED as claimed in claim 8, wherein the polyimide film, the copper foil or the copper alloy foil, and the aluminum foil or the aluminum alloy foil are bonded together using a hot-press forming machine. 